Data Flow Diagram (DFD)

A DFD is a model expressing what data flow to and from what processes. It clarifies both (sub) processes and data flows.
At level 0, the context diagram, the system boundaries, scope is pictured:
DFD Context diagram
At level 1, the main processes in the system are visualized:
DFD level 1
When required, lower level give even a more detailed view.